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Three Watt Luxeon Bike light.

Looking for a better light.

I'm always on the lookout for a better bike light. I have built lights based around halogen lamps, though at the moment I do most of my night riding with a set of Vistalite ten and fifteen watt commercially built units. Their 2.2 amp hour Nickel Metal Hydride batteries are good for rides of up to an hour, depending on whether I use a 10 watt or a 15 watt lamp head.

First came the one watt Luxeons. These were good but rather expensive and not bright enough to be practical for something like cycling in the dark. Five watt Luxeons followed a year or so later and these were good but very
hot, very expensive and not particularly reliable.

In 2004, Lumileds released their three watt Luxeons. Might these make practical bike lights?

Anxious not to reinvent the wheel, I did some web surfing to see if anyone else had made a decent Luxeon-based bike light. Via the Candlepower Forums I found that a chap calling himself Elektrolumens had done some experimenting, but I didn't find any information that I could use.

Luxeon emitters have not been very widely distributed in Australia until recently. When I found that Prime Electronics had started selling them, I bit the bullet and ordered some. I decided to start with a pair of three watt Luxeon Stars and a pair of 'narrow' beam lens assemblies.

Prime had three types of collimating lenses available to suit Luxeon Stars. I chose to try out the one with the narrowest beam angle for road riding at night.

Design.

Here's what the bare three watt Luxeon Star looks like. The image doesn't really indicate its size. This one is about 20mm across the outer edges.

A Luxeon star is a LED emitter bonded
to a small piece of circuit board material with an aluminium backing
designed to dissipate heat. In operation, Luxeons get hot, so the
aluminium needs to be in contact with something that can conduct heat
away from the emitter.

Here is what the collimating lens and reflector looks like.

So with these two pieces of hardware on hand, I set about designing a way to use them in a bike light. The first thing I did was visit a local bike shop to get some plastic mounting brackets to fit handlebars. These are often supplied with reflectors for new bikes and bike shops may either give them away or charge a nominal fee for them. On this occasion a pair of brackets complete with reflectors that I didn't need, cost me two dollars.

Next I took a Luxeon lens assembly with me and went to visit a local distributor of industrial aluminium supplies. I reasoned that since aluminium is an easy material to work with, is quite sturdy and conducts heat well, it would make an ideal housing for a light. Unfortunately none of the standard sizes of aluminium tubing could be found to readily accommodate the lens, so I had to go and have a re-think.

Just down the road from the purveyor of fine aluminium products, I found one of those giant warehouse-sized hardware places. Lens in hand, I commenced wandering its vast aisles in search of anything vaguely housing-like. My browsing eventually brought me to the plumbing products, where I found a thing called a 25mm PVC coupling.

It may as well have been purposely designed to fit the lens assembly.

A great fit, and cheap at $1.70 each.

While the PVC would make a fine housing, it's a terrible conductor of heat, so with PVC coupling in hand I set off to the local electronics shop in search of a heatsink. I didn't need to look far before I encountered one of these...

...and found that it fit the coupling just like this.

The heatsink is a fairly standard TO-3 transistor heatsink and the fine folks at Jaycar Electronics charged me $3.75 each for a couple of them.

Construction.

Putting all the pieces together didn't take long. So that the plastic mounting bracket could be bolted to the PVC coupling to mount the whole thing on a bike, I cut off a couple of heatsink fins that would get in the way. To let a bit more light spill down to the road from the lens (and to make it all look more like a lamp) I cut away some of the 'front' of the PVC coupling with a hacksaw.

A bit of hot melt glue holds the lens assembly securely inside the PVC coupling. I had intended to just let the back of the lens wedge the Luxeon in place on the heatsink, but this proved too unstable and hard to line up. Instead the Luxeon is bolted to the heatsink with a pair of electrically insulating nylon bolts and a smear of heat transfer compound to thermally bond it to the heatsink surface.

The heatsink is fixed to the PVC coupling with three small self tapping screws.

The curly cord that connects the Luxeon to the outside world (and ultimately its battery pack) is one from a big box of curly cords I bought in a disposal sale a few years ago. The two wires in the cord are actually too small in cross section for this application, and together they have an end to end series resistance of about half an ohm.

At the 700~1000mA that this 3 watt Luxeon will draw, half an ohm of cable resistance will result in a significant voltage drop between the batteries and the light. Normally I would take pains to avoid such a drop, but in this situation it turns out to be a benefit.

In the short term I intend to power the lights from non-rechargeable batteries. A good fit for the job would appear to be C size alkaline cells which have a capacity of over 8000 milliamp hours. Three C cells in series will nominally deliver 4.5 volts. A 3 watt Luxeon needs about 3.7 volts. To avoid damaging the Luxeons, I should introduce some form of current-limiting circuit. Fortunately, the resistance of the curly cord combined with the naturally high internal resistance of alkaline cells results in a light that works perfectly without any additional electronics.

Here's an upside down close up of the prototype.

Once I have the batteries and lamp mounted on a bike, I'll try to get some night photos of the lamp's performance.

If a Luxeon draws 1000mA from an 8000mAh battery, that's 8 theoretical hours of light. Real life batteries don't work like that, but given the ability of LEDs to produce useful light deep into the battery's discharge, I'm hoping for at least 5 or 6 hours of good light out of three alkaline C cells.

Update: Night tests

Having lashed up a very rough battery holder for three C size alkaline cells, I attached the lot to my bike and rolled it out into the dark for a look. For comparison purposes, I also set up a 10 watt Vistalite with a charged 2.2Ah Ni-MH battery.

A VHS video cassette case sacrificed itself to become a battery holder. Yes, that is a toe strap holding it against the head tube.

The plastic clamp holds the lamp in place on the handlebars.

Photographs of the comparison between the 10 watt halogen and the 3 watt Luxeon are yet to show a meaningful difference because I can only control the Aperture OR Shutter speed on my digital camera. The various white balance adjustments also fail to show the difference between the Luxeon's very white light and the halogen's (hitherto unnoticed) yellow light.

What is clear is that the Luxeon is quite bright and very white. It is not ultimately as bright as the 10 watt halogen and I certainly wouldn't expect it to be.

This photo shows the bike leaning against the garage and shining the Luxeon on a fence while the silly-bright 50,000 mCd rear flasher does its best to blind the camera.